Eighteen celestial objects were caught loitering
recently in a young stellar cluster near Orion’s belt, rather than orbiting
stars as most planets do, reports a team of astronomers in the Oct. 6 Science.
The astronomical community is divided over what to do with these misfit
youths.

Based on our own solar system, a massive body
is defined as a planet if it has no nuclear reaction in its interior and
orbits a star, says Maria Rosa Zapatero Osorio, who led the team at the
Astrophysics Institute of the Canary Islands. But that definition leaves
the new-found free-floaters, which have planet-like interiors, in semantic
limbo. They hover in the star cluster without orbiting anything.

While some astronomers would like to initiate
a change in policy to allow planets the freedom to drift when young, “most
astronomers still insist that planets have to be in orbit about stars,”
says Jibor Basri of the University of California at Berkeley.

At five to 15 times the mass of Jupiter, these
new kind of isolated giant planets, as the Canary Island team calls them,
breach the size of brown dwarfs. A failed star, a brown dwarf burns deuterium
in its core and not hydrogen, as other stars do. It must be at least 13
times the mass of Jupiter before it can burn deuterium, but never reach
75 times Jupiter’s mass, the point at which hydrogen fusion begins.

Location of the Orionis star
cluster (region around the brightest
star in thebottom right side of the figure)
and finding charts (45” by 45”)
for three spectroscopically confirmed,
very young cluster membranes with
massesbetween 5 to 15 times the mass
of Jupiter.

And at three orders of magnitude younger than
our sun, these strange objects are challenging the theory of how planetary
bodies form. One theory might have the objects forming after the breakup
of a large molecular cloud as the smaller fragments collapse under their
own gravity, similar to the way brown dwarfs form, Zapatero Osorio says.

“No matter what terminology astronomers eventually
settle on, it’s already clear that the study of free floating, low-mass
objects — stars, brown dwarfs, and now planetary mass objects — can lend
significant insight to our understanding of how stars and planets form,”
says Joan Najita of the National Optical Astronomical Observatories. “I
think that will be the ‘legacy’, so to speak, of the reported observations.”